The present invention relates to an optical device; and, more particularly, to a current blocking layer structure of an optical device having a buried ridge structure (BRS) and a fabrication method thereof.
Generally, optical devices used for an optical fiber communication, e.g., a high-performance semiconductor laser diode (LD) or a semiconductor optical amplifier (SOA), requires a low threshold current and high quantum efficiency, and for this, leakage current should be minimized.
To minimize the leakage current, semiconductor lasers using a planar buried heterostructure (PBH) and a buried ridge structure (BRS) are used. The optical source and the optical amplifier used in the optical fiber communication need to be high-powered, highly reliable and perform at a high speed. Therefore, it is necessary to optimize a current blocking layer used in these structures.
To form such a current blocking layer, three methods are used commonly.
The first method is what uses a P-N-P current blocking structure. It is based on a principle that when an electric current is supplied to an electrode, the current is blocked because other parts except an active layer is supplied with a reverse bias, while the active-layer receives a forward bias. This is a first conventional method to be described below.
Secondly, there is a method that grows a semi-insulation layer and uses it as a current blocking layer. As for the semi-insulation layer, iron (Fe) is usually doped and used. This is a second conventional method that will be also described later on.
The third conventional method is to implant hydrogen ions into the parts except the active layer and block a current.
 less than Conventional Method 1 greater than 
FIGS. 1A to 1E are cross-sectional views illustrating a first method for fabricating a conventional optical device of a planar buried heterostructure having a P-N-P current blocking layer.
First, as described in FIG. 1A, an active layer 102 of a heterostructure and a p-InP layer 103 are grown on an n-InP substrate 101 with a liquid phase epitaxy (LPE) or a metalorganic chemical vapor deposition (MOCVD), and an insulation layer mask 104 for a mesa-etching is formed on top of the p-InP layer 103 in a photolithography etching process.
Subsequently, as shown in FIG. 1B, the p-InP layer 103, the active layer 102 and a part of the n-InP substrate 101 are etched by using the insulation layer mask 104, but a mesa structure is formed through the processes of dry-etching and wet-etching.
Subsequently, as depicted in FIG. 1C, to confine the current and optical ray of the active layer 102, a p-InP 105 and an n-InP 106 are grown again in the LPE or MOCVD method.
Subsequently, as illustrated in FIG. 1D, after removing the insulation layer mask 104 used for the mesa-etching, a p-InGaAs layer 108 for ohm(xcexa9)-contacting with a p-InP clad layer 107 is grown again in the LPE or the MOCVD method.
Subsequently, as described in FIG. 1E, the region overlapped with the upper part of the active layer 102 is open by depositing an insulation layer 109 on the p-InGaAs layer 108 and performing a photolithography etching process, and a p-electrode 110 and an n-electrode 111 are formed to contact with the p-InGaAs layer 108 and the n-InP substrate 101, respectively.
In the first conventional method described above, in case of a one-layer waveguide, the good static characteristics can be obtained by re-growing the P-N-P. However, in case that the waveguide is of complicated structure with two or more layers in the vertical direction for the integration of an optical device, it is not only hard to re-grow the P-N-P current blocking structure, but a problem of increasing leakage current is raised, even if the P-N-P is re-grown.
 less than Conventional Method 2 greater than 
FIGS. 2A to 2E are cross-sectional views describing a second method for fabricating a conventional optical device of a planar buried heterostructure having a semi-insulation current blocking layer.
First, as illustrated in FIG. 2A, an active layer 202 and a p-InP layer 203 are grown on an n-InP substrate 201 in a liquid phase epitaxy (LPE) or a metalorganic chemical vapor deposition (MOCVD) method, and an insulation layer mask 204 for the mesa-etching is formed on top of the p-InP layer 203 in a photolithography etching process.
Subsequently, as shown in FIG. 2B, the p-InP layer 203, the active layer 202 and a part of the n-InP substrate 201 are etched by using the insulation layer mask 204, but a mesa structure is formed through the processes of dry-etching and wet-etching.
Subsequently, as depicted in FIG. 2C, to confine the current and optical ray of the active layer 202, a semi-insulation InP current blocking layer 205 is grown again in the LPE or MOCVD method.
Subsequently, as illustrated in FIG. 2D, after removing the insulation layer mask 204 used for the mesa-etching, a p-InGaAs layer 207 for ohm(xcexa9)-contacting with a p-InP clad layer 206 is grown again in the LPE or the MOCVD method.
Subsequently, as described in FIG. 2E, the region overlapped with the upper part of the active layer 202 is open by depositing an insulation layer 208 on the p-InGaAs layer 207 and performing a photolithography etching process, and a p-electrode 209 and an n-electrode 210 are formed to contact with the p-InGaAs layer 207 and the n-InP substrate 201, respectively.
The second conventional method described above has advantages that it solves the problem of increasing parasitic capacitance, which is caused in the aforedescribed first conventional method, and improves the property of high-speed modulation, and that it simplifies the processes.
However, since the semi-insulation layer used as a current blocking layer works as a deep level center in which holes and electrons are combined, it is more likely to work as a path for leakage current than as a current blocking layer. Also, there is a problem that the performance of the active layer is degraded due to the diffusion of iron (Fe), which is used as a doping impurity for forming a semi-insulation layer.
 less than Conventional Method 3 greater than 
FIGS. 3A to 3E are cross-sectional views showing a third method for fabricating a conventional optical device of a buried ridge structure having a current blocking layer using a hydrogen ion implantation method.
First, as illustrated in FIG. 3A, an active layer 302 and a p-InP layer 303 are grown on an n-InP substrate 301 in a liquid phase epitaxy (LPE) or a metalorganic chemical vapor deposition (MOCVD) method, and an insulation layer mask 304 for the mesa-etching is formed on top of the p-InP layer 303 in a photolithography etching process.
Subsequently, as shown in FIG. 3B, the p-InP layer 303, the active layer 302 and a part of the n-InP substrate 301 are etched by using the insulation layer mask 304, but a mesa structure is formed through the processes of dry-etching and wet-etching.
Subsequently, as depicted in FIG. 3C, to confine the current and optical ray of the active layer 302, the insulation layer mask 304 is removed and a p-InGaAs layer 306 is grown again to ohm-contact with a p-InP clad layer 305 in the LPE or MOCVD method.
Subsequently, as illustrated in FIG. 3D, to form a current blocking layer using hydrogen ion implantation, a hydrogen ion blocking mask 307 is formed overlapped with the active layer 302 in a photolithography etching process, and a current blocking layer 308 is formed by using the hydrogen ion implantation method.
Subsequently, as described in FIG. 3E, after removing the hydrogen ion blocking mask 307, a region overlapped with the upper part of the active layer 302 is open by depositing an insulation layer 309 on the p-InGaAs layer 306 and performing a photolithography etching process, and a p-electrode 310 and an n-electrode 311 are formed to contact with the p-InGaAs layer 306 and the n-InP substrate 301, respectively.
The third conventional method described above has an advantage of simplifying the fabrication process.
However, since it uses hydrogen ions injected to the current blocking layer, the width of the current blocking layer by the implantation of the hydrogen ions is 10 xcexcm, whereas the width of the active layer is 1xcx9c2 xcexcm, and thus the p-clad layer surrounds the active layer. This has a positive effect that the current and optical ray is confined in the active layer as well as a negative effect that it works as a path for leakage current.
It is, therefore, an object of the present invention to provide an optical device having a current blocking structure that can be favorably applied to a multi-layer waveguide structure as well as an advantage of low leakage current.
It is another object of the present invention to provide a method for fabricating an optical device, which can reproduce optical devices relatively comfortably.
In accordance with an aspect of the present invention, there is provided an optical device, including: active layers of a mesa structure in a predetermined region on a substrate; a first current blocking layer of a P-N-P structure, which is placed to cover the mesa structure; and a second current blocking layer of a buried ridge structure, which is placed to surround the environs of the first current blocking layer.
In accordance with another aspect of the present invention, there is provided a method for fabricating an optical device, including the steps of: a) forming active layers of a mesa structure in a predetermined region on a substrate; b) forming a first clad layer having a p-type along the surface of the active layers; c) forming a second clad layer having a n-type on the first clad layer by using the favorable condition of predominant side growth; d) exposing the first clad layer in the upper part of the active layers by etching the second clad layer; e) forming and planarizing a first conductive third clad layer in the upper part of the whole structure completed with the above four steps; and f) forming a current blocking layer by hydrogen ion implantation on the first clad layer and the second clad layer.